1. Field of the Invention
This invention relates to a novel non-woven electret fiber sheet useful, for example, as a filter, and to a process and apparatus for producing such a sheet which, even after loss of its electret properties, still retains adequate filtration properties by virtue of the uniform dispersion of its constituent fibers provided by the process and apparatus.
2. Description of Prior Art
A process of producing an electret sheet consisting of extremely thin fibers is disclosed in U.S. Pat. No. 4,215,682.
The process comprises subjecting a molten polymer to a melt-blow spinning, rendering the formed fibers electret (electretizing) immediately after the melt-blow spinning by passing them through an electrical field formed between two non-contact voltage-application electrodes disposed near the spinneret, and collecting the electret fibers on a collecting apparatus. In this process, electretizing is started after spinning and completed before the collection of the fibers.
However, this process has several drawbacks.
A. Since the electric charge is applied to the fibers without contacting them with an electrode, that is, the electric field acts on the fibers flying in space, a sufficient amount of electric charge cannot be trapped within the fibers.
B. The amount of trapped charge in the sheet is not large since, perhaps, the direction of polarity of each fiber is randomized before collection and the sheet as a whole has almost no polarity.
C. The flying speed of the melt-blown fibers is as high as 150 m/second. Thus the fibers stay in the electric field only for an instant, sufficient amount of electric charge cannot be trapped.
D. Since the fibers electretized immediately after the melt-blow spinning, part of the electric charge escapes due to the thermal movement of the component molecule of the fibers.
Because of the above drawbacks, the amount of the electric charge trapped in the electret sheet is small, and the stability of the electret is poor.
U.S. Pat. No. 4,592,815 discloses a process of producing an electret non-woven sheet comprising placing a non-woven sheet on a grounded plate, and subjecting the non-woven sheet to corona discharge thereby trapping electric charge in the non-woven sheet and causing ionic depolarization.
Similarly, EP-A-0182512, which was published after the priority date of the present application, describes a process for producing an electret fiber sheet in which a preformed fiber sheet having a covering factor of at least 60% is placed between mutually non-contact and earth electrodes and a voltage of at least 3 KV/cm, at an electric current density of at least 1.times.10.sup.-6 mA/cm.sup.2, is applied between the electrodes.
However, for the reasons described below, these process are applicable only to the formation of sheets of relatively low thickness and therefore relatively low weight. Thus the process of EP-A-0182512 is limited for the production of electret fiber sheets having a weight no more than 80 g/m.sup.2.
EP-A-0182512 also contains a general disclosure of electret fiber sheets having a total polarized charge per unit area of at least 7.times.10.sup.-11 Coulomb/cm.sup.2, but no disclosure that this applies to sheets of all disclosed weights especially those of highest weight.
Thus, processes such as those described in U.S. Pat. No. 4,592,815 and EP-A-018512 suffer from the disadvantage that the electretic character of the sheet is discreased as the weight of the sheet increases. This is because, although a large amount of electric charge generated by the corona discharge is implanted into the surface portion of the sheet, the relative amount of the electric charge reaching the internal portion of the sheet becomes small since the surface portion acts as a barrier.
Another problem encountered in a process of producing a non-woven sheet (whether it is an electret sheet or not) by melt-blow spinning is that if the weight of the collected fibers increases, the amount of air flowing through the wire net is decreased, so that the air flow is reflected by the collecting surface thus causing a turbulent flow. This brings about an irregularity in the weight of the sheet and tangles the fibers.
The invention seeks to provide a solution to such problems.